Valian year?

[Mumriken]

they would eventually pass away (die at their own desire and of free will

Sounds to me like "EVENTUALLY" and "FREE WILL" suggests their life would be prolonged. How many years we are talking about...who knows. I'd say a hundred or so.

[Galin]

All that need mean (in my opinion) is that eventually their time would come to die (whether brief or long), but knowing this they could then choose to die of free will instead of waiting untill the moment death would naturally take them.

As for Frodo or other mortals, they could only dwell in Aman for a limited time - whether brief or long. The Valar had neither the power nor the right to confer 'immortality' upon them. Their sojourn was a 'purgatory', but one of peace and healing and they would eventually pass away (die at their own desire and of free will) to destinations of which the Elves knew nothing.

The Valar have no right to confer immortality upon them... but did have the right to extend their life span? For myself I don't think so; again I just think Tolkien is comparing their deaths to that of Aragorn.